Astrocytes play an important role in the maintenance of normal brain activity as well as the brain development. Astrocytes have been reported to help the neuron activity by removing released neurotransmitters or adjusting the ion concentration in a brain during last several decades. Recently, astrocyts are involved in the synapse formation, the control of number of synapse, and flexibility of synapse, and also functions in the differentiation of nerve stem cell to nerve tissue.
Glutamate is the principal excitatory neurotransmitter in the brain. In the central nervous system, both neurons and astrocytes are known to release glutamate; the neuronal glutamate mediates fast synaptic transmission, whereas the astrocytic glutamate appears to modulate synaptic transmission. While it is clear that neurons release glutamate via Ca2+-dependent exocytosis, the mechanism of astrocytic glutamate release has been heavily debated and many controversial issues have been raised.
The mechanism on the release of astrocytic seems to be divided into three conflicting views: (1) astrocytes release glutamate by vesicular exocytosis just like neurons (Bezzi et al., Nat Neurosci 7, 613-620, 2004; Jourdain et al., Nat Neurosci 10, 331-339, 2007), (2) Ca2+-dependent astrocytic glutamate does not do anything important (Agulhon et al., Science 327, 1250-1254, 2010), or (3) glutamate is released from astrocytes by transporters or channels (Cavelier and Attwell, J Physiol 564, 397-410, 2005), molecular mechanism of release has been controversial.